A valve which provides flow ports between the internal and external portions that are selectively isolated until a successful integrity test of the casing or tubing string is completed. Hydrostatic pressure applied to the internal portion of the tool causes shifting of an internal piston which compresses gasses until a limit is reached which permanently opens the valve.
Said valve may be conversely used in any application in which hyrodstatic pressure is the preferred or only available method of opening a valve.